Line a small plate or baking sheet with wax paper. Scoop 12 teaspoons of Nutella onto the plate and freeze for 15–20 minutes until firm.
Preheat oven to 375°F and line a 12-cup muffin pan with paper liners.
In a large bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.
In a separate bowl, whisk together sugars, eggs, buttermilk, oil, and vanilla until smooth.
Add wet ingredients to dry and stir just until combined with a rubber spatula.
Fill each muffin liner halfway. Place a frozen Nutella scoop in the center of each. Top with remaining batter until about ¾ full.
Warm remaining Nutella slightly (about 10 seconds in the microwave) and spoon ½ teaspoon over the top of each muffin. Swirl gently with a toothpick. Sprinkle muffin tops with sanding sugar if desired.
Bake for 16–18 minutes or until muffin tops are set and a toothpick inserted beside the center comes out clean.
Cool muffins in the pan for 5–10 minutes, then transfer to a wire rack. Serve warm if desired.